Through various studies these days, the case where fine irregular structures having a period equal to or less than that of a visible light wavelength range (about 380 to 780 nm) are formed on the surface of optical devices such as displays, light-emitting diodes, solar cells, etc. is known to exhibit an antireflective effect and a lotus effect to thus increase the efficiency of optical devices. Such a fine irregular structure, which is called a moth-eye structure, plays a role in buffering changes in refractive index between two media having different refractive indexes. Specifically, when light passes through two different media, reflection occurs due to the difference in refractive index, but the refractive indexes of two media are continuously increased in the presence of the fine irregularity between the two media, thus preventing such reflection.
The formation of a fine irregular structure on the surface of an optical device may include, for example, the method including the following steps of (i) to (iii) (nano-imprinting):
(i) supplying an active energy ray-curable composition between a mold having a structure complementary to a fine irregular structure and a substrate film, which is the base of a light-transmitting film;
(ii) forming a cured resin layer having a fine irregular structure on the surface of the substrate film by curing the active energy ray-curable composition through irradiation with an active energy ray such as UV light or the like; and
(iii) separating the mold from the cured resin layer.
However, the mold has pores having a period on the nanometer scale and a relatively large aspect ratio, and thus the contact interface between the mold and the active energy ray-curable composition becomes large. Thereby, it is difficult to accurately imprint the pattern of the mold on the cured resin layer, and the separation of the mold in step (iii) becomes very difficult with an increase in the interfacial force. In particular, since the separation of the mold is directly associated with productivity, some patents for solving this problem have been disclosed.
In this regard, Japanese Patent Application Publication No. 2007-326367 discloses a method of treating a surface, having a fine irregular structure, of a mold with a release agent (an external release agent), and Japanese Patent Application Publication No. 2009-061628 discloses the use of a light-curable transfer layer in a solid phase comprising a light-curable resin composition including a phosphoric acid ester-based compound as an internal release agent. However, in the case where only the treatment with the release agent is simply performed, as in the above patents, releasability may gradually decrease due to the repeated transfer process, or the surface of the mold may become contaminated due to the deposition of the release agent.
Meanwhile, the substrate film on which the cured resin layer having a fine irregular structure is formed is made of PE, PC, PMMA, etc., and such a substrate film has high transparency and flexibility but is disadvantageous in that it is prone to wrinkling and curling. Also, properties such as strength and the like may be improved due to the use of the substrate film, but it is difficult to form a thin film, and the reduction in the manufacturing cost is limited.
Moreover, as shown in FIG. 1, a multilayer structure including a substrate film and a cured resin layer is conventionally formed, and thus light transmission loss may occur at the interlayer interface and non-transmitted light may act as a cause of reflection. Therefore, techniques that do not use the substrate layer have been devised, but are merely implemented in a manner in which the substrate film is stripped through chemical treatment after the formation of the resin layer, and a pattern to be formed is limited to a micro scale.